1. Field of The Invention
The present invention relates to an apparatus for treating the surface of a semiconductor substrate and a method of controlling the apparatus. More particularly, it pertains to an apparatus which performs a surface treatment, such as cleaning or etching, of an oxide film and the like on the surface of a semiconductor wafer, and to a method of controlling such an apparatus.
2. Description of Related Art
FIG. 7 is a schematic view showing the structure of a conventional apparatus for etching a surface, such as in cleaning, of a semiconductor substrate (hereinafter referred to as a surface treatment apparatus). In FIG. 7, an HF aqueous solution 7 prepared by dissolving hydrogen fluoride (HF) in water (H.sub.2 O) is contained in an HF aqueous solution tank 2. A carrier gas pipe 1 for introducing a carrier gas, such as an inactive gas like nitrogen gas, is disposed over the HF aqueous solution tank 2. A carrier gas cylinder 1a is connected to the carrier gas pipe 1 through a carrier gas valve 1b. A temperature adjusting device 12 is provided around the HF aqueous solution tank 2. Temperature control means 13 maintains the temperature of the HF aqueous solution 7 in the tank 2 at a fixed value, so that the vapor pressure of the HF aqueous solution 7 is maintained at a constant level, whereby a fixed amount of the HF aqueous solution 7 is vaporized. A vapor 7a of the HF aqueous solution is conveyed by the carrier gas which is fed through the carrier gas pipe 1, so it is then introduced into a reaction chamber 4 through a pipe 3 (hereinafter referred to as a vapor pipe). The reaction chamber 4 is provided with an exhaust pipe 6 for exhausting gas there-from.
The conventional surface treatment apparatus is constructed as described above. The vapor 7a of the HF aqueous solution 7 is introduced by the carrier gas into the reaction chamber 4. The carrier gas is fed into the HF aqueous solution tank 2 through the carrier gas pipe 1. Because HF is dissolved in water, the HF aqueous solution is ionized as follows: EQU H.sub.2 O.fwdarw.H.sup.+ (H.sub.3 O.sup.+)+F.sup.-
The vapor of the ionized HF aqueous solution flows to a silicon wafer 8 on a wafer stand 5. As shown in FIG. 8, the vapor reacts with an oxide film 10, such as SiO.sub.2, with which device structures 9 are covered, so as to remove the oxide film 10.
The above-described surface treatment apparatus removes the oxide film 10 in the following manner. First, SiO.sub.2 reacts with HF vapor as expressed by the following reaction equation (1): EQU SiO.sub.2 +6HF=H.sub.2 SiF.sub.6 +2H.sub.2 O (1)
Then, H.sub.2 SiF.sub.6 generated by reaction formula (1) decomposes into SiF.sub.4 and 2HF as expressed by the following reaction equation (2): EQU H.sub.2 SiF.sub.6 =SiF.sub.4 +2HF (2)
SiF.sub.4 is usually a gaseous substance at the temperature at which surface treatments, such as cleaning and etching, are performed, and is discharged through the exhaust pipe 6. However, because the solvent which dissolves HF is water and furthermore H.sub.2 O is generated by reaction formula (1), before SiF.sub.4 is removed from the surface of the silicon wafer 8, it is reacted with H.sub.2 O, as expressed by the following reaction formula (3): EQU 3SiF.sub.4 +H.sub.2 O=Si(OH).sub.4 +2H.sub.2 SiF.sub.6 ( 3)
This reaction generates reaction products, such as Si(OH).sub.4 and H.sub.2 SiF.sub.6, which are residues.
The same reaction products as those just mentioned above are also generated when tetraethyl orthosilicate (TEOS) is removed from the surface of a semiconductor wafer.
As shown in FIG. 9, such reaction products remain as a residue 11 between the device structures 9 on the silicon wafer 8. Conventionally, the residue 11 is first treated with water and a solvent, such as isopropyl alcohol, to remove it. However, water and isopropyl alcohol are liquids which do not enter completely into the gap between the device structures 9, and therefore the residue 11 cannot be removed completely.